1. Field of the Invention
The present invention relates to a flash EPROM, which can be rewritten electrically.
2. Description of the Prior Art
In the non-volatile memory semiconductor in the prior art, for example, a flash EPROM, it is reported that stored charges gradually discharge in a long time due to a minute defect in a memory cell. As a result, the stored data changes so that a data error occurs.
For avoiding the occurrence of such a data error, the non-volatile memory semiconductors in the prior art are screened before their shipment, and the reliability of data to be stored therein is guaranteed for a certain period of the utilization.
Some users, who use non-volatile memories, such as a flash EPROM, in a very hard condition, desire that data can be stored with high reliability at least in a certain memory area in the EPROM, and they admit the smallness of the memory size of such a high reliability memory area, if such reliability can be assured.
FIG. 10 is a block diagram of a flash EPROM in the prior art. The Flash EPROM comprises memory areas 101-103, a selector 104, a sense amplifier 105, and word line decoders 106-108 for each of the memory areas 101-103.
The function of the flash EPROM is explained below.
When the flash EPROM receives an address signal sent from an outer circuit, the word line decoders 106-108 decode the signal to select a word line, on the basis of the address signal.
The selector 104 selects a bit line. The data stored in the memory element at the crossing point of the bit line selected by the selector 104 and the word line selected by the word line decoders is read out by the sense amplifier 105 and is outputted to the outer circuit of the flash EPROM.
Each of the independent memory areas 101-103 in the non-volatile memory semiconductor in the prior art can independently write, store, and read data. However, means for verify whether the data read out from the memory areas 101-103 is correct or not is not equipped. Thus, there is a problem in the reliability of the read out data.